Pharmacokinetic Significance of Renal OAT3 (SLC22A8) for Anionic Drug Elimination in Patients with Mesangial Proliferative Glomerulonephritis

Interactions of human organic anion transporters 1-4 and human organic cation transporters 1-3 with the stimulant drug methamphetamine and amphetamine

Drug membrane transport system proteins, namely, drug transporters, are expressed in the kidney and liver and play a crucial role in the excretion process. This study aimed to elucidate the interactions of the drug transporters human organic anion transporters 1, 2, 3, 4 (hOAT1, 2, 3, 4) and human organic cation transporters 1, 2, 3 (hOCT1, 2, 3), which are expressed primarily in human kidney, liver, and brain, with the stimulants methamphetamine (METH) and amphetamine (AMP). The results of an inhibition study using representative substrates of hOATs and hOCTs showed that METH and AMP significantly inhibited (by >50%) uptake of the hOCT1 and hOCT3 representative substrate 1-methy1-4-phenylpyridinium ion (MPP+) and hOCT2 representative substrate tetraethyl ammonium (TEA). However, METH and AMP did not inhibit uptake of the representative substrates of hOAT1, hOAT2, hOAT3, and hOAT4, (i.e., p-aminohippuric (PAH) acid, prostaglandin F2α (PGF2α), estron sulfate (ES), and ES respectively). Kinetic analyses revealed that METH competitively inhibited hOCT1-mediated MPP+ and hOCT2-mediated TEA uptake (Ki, 16.9 and 78.6 µM, respectively). Similarly, AMP exhibited competitive inhibition, with Ki values of 78.6 and 42.8 µM, respectively. In contrast, hOCT3 exhibited mixed inhibition of representative substrate uptake; hence, calculating Ki values was not possible. Herein, we reveal that hOCTs mediate the inhibition of METH and AMP. The results of this uptake study suggest that METH and AMP bind specifically to hOCT1 and hOCT2 without passing through the cell membrane, with subsequent passage of METH and AMP via hOCT3.

RNA-seq of serial kidney biopsies obtained during progression of chronic kidney disease from dogs with X-linked hereditary nephropathy

Dogs with X-linked hereditary nephropathy (XLHN) have a glomerular basement membrane defect that leads to progressive juvenile-onset renal failure. Their disease is analogous to Alport syndrome in humans, and they also serve as a good model of progressive chronic kidney disease (CKD). However, the gene expression profile that affects progression in this disease has only been partially characterized. To help fill this gap, we used RNA sequencing to identify differentially expressed genes (DEGs), over-represented pathways, and upstream regulators that contribute to kidney disease progression. Total RNA from kidney biopsies was isolated at 3 clinical time points from 3 males with rapidly-progressing CKD, 3 males with slowly-progressing CKD, and 2 age-matched controls. We identified 70 DEGs by comparing rapid and slow groups at specific time points. Based on time course analysis, 1,947 DEGs were identified over the 3 time points revealing upregulation of inflammatory pathways: integrin signaling, T cell activation, and chemokine and cytokine signaling pathways. T cell infiltration was verified by immunohistochemistry. TGF-β1 was identified as the primary upstream regulator. These results provide new insights into the underlying molecular mechanisms of disease progression in XLHN, and the identified DEGs can be potential biomarkers and therapeutic targets translatable to all CKDs.

Valacyclovir-Induced Acute Kidney Injury in Japanese Patients Based on the PMDA Adverse Drug Reactions Reporting Database

BACKGROUND

More than 250,000 reports of adverse drug events were included in the database of the Pharmaceuticals and Medical Devices Agency (PMDA) in Japan. However, these data have not been utilized sufficiently for analysis. While valacyclovir is the antiviral agent used worldwide, it is reported to induce nephrotoxicity. The aim of this study was to clarify the profiles of valacyclovir-induced adverse events using the PMDA database.

METHODS

Case reports were screened in the PMDA adverse event database from 2004 to 2011. The profiles of patients with acute kidney injury (AKI) were analyzed by sex, age, diseases, concomitant suspected drugs, and outcomes.

RESULTS

A total of 514 kidney-related adverse events were detected, and 344 were cases that included AKI. Of the AKI cases, 246 patients (71.5%) were female. There were 145 patients who were 70 to 79 years of age, which was the most affected of all age groups. Of the 344 patients, 183 patients had hypertension, and 65 had diabetes. Valacyclovir was the only drug used among 257 patients (74.1%).

CONCLUSIONS

There were many reports of AKI involving valacyclovir and females, particularly in the 70- to 79-year age group in Japan. The results suggest that these patients were most likely to develop AKI after valacyclovir treatment.

Interactions of human organic anion transporter 1 (hOAT1) with substances associated with forensic toxicology

Renal excretion is an important elimination pathway for substances associated with forensic toxicology, such as medicines, agricultural chemicals, and industrial chemicals. This study aimed to elucidate the renal elimination pathway of substances using culture cells stably expressing the human organic anion transporter 1 (hOAT1) gene. Substances tested were diazepam, triazolam, haloperidol, amitriptyline, mianserin, bromovalerylurea, phenobarbital, acetaminophen, acetylsalicylic acid, lidocaine, aconitine, atropine, caffeine, nicotine, malathion, dichlorvos, fenitrothion, chlorpyrifosmethyl, paraquat, diquat, potassium cyanide, sodium arsenite, sodium azide, o-cresol, and probenecid (control, a representative inhibitor of hOAT1). Results demonstrated that diazepam, triazolam, amitriptyline, mianserin, malathion, fenitrothion, chlorpyrifosmethyl, and probenecid significantly inhibited representative substrates of hOAT1 and para-aminohippuric acid uptake by hOAT1. IC(50) values of the aforementioned substances were 133.3, 185.2, 354.1, 312.6, 114.2, 26.6, 191.5, and 7.9μM, respectively. Ki values were 83.5, 86.0, 573.9, 99.0, 134.0, 51.2, 324.6, and 9.1μM, respectively. In conclusion, the current results suggest that fenitrothion and chlorpyrifosmethyl are transported with pharmacokinetics indicative of hOAT1 involvement in the human kidney.

Effects of metabolic acidosis on expression levels of renal drug transporters

PURPOSE

In the renal proximal tubular cells, various transporters play important roles in the secretion and reabsorption of drugs. When metabolic acidosis is induced, a number of adaptive changes occur in the kidney. The purpose of this study was to clarify the changes of drug transporters under the acidosis and the effects of these changes on urinary drug excretion.

METHODS

Wistar/ST rats were given 1.5% NH₄Cl in tap water for 48 h to induce the acidosis. Pharmacokinetics of PSP or metformin was evaluated. In addition, expression levels of drug transporters were examined by Western Blotting.

RESULTS

The renal clearance of PSP was markedly decreased, whereas the creatinine clearance and renal clearance of metformin were unchanged. Furthermore, Western blots indicated that the protein expression level of organic anion transporter (OAT) 3 was decreased. In contrast to OAT3 levels, OAT1 and organic cation transporter (OCT) 2 levels were unaffected. An immunohistochemical analysis showed that the OAT3 protein in the proximal tubules was localized in the basolateral membrane both of the normal and the acidosis rats.

CONCLUSION

The decrease of renal excretion of anionic drugs during metabolic acidosis might be partly due to a reduction in the level of OAT3 protein.

Hepatitis C virus-related cirrhosis is a major determinant of the expression levels of hepatic drug transporters

Hepatic drug transporters are responsible for both hepatic uptake and the biliary excretion of drugs. Expression changes in hepatic drug transporter genes have been observed in various pathophysiological conditions. However, it has not been comprehensively investigated what factors substantially influence the mRNA levels of hepatic drug transporters. In this study, we quantified the mRNA expression of 17 drug transporters using noncancerous liver tissue samples and carried out stepwise multiple regression analysis to identify the factors affecting their expression from 18 clinical variables. For 17 drug transporters, the mRNA level of organic anion transporting polypeptide (OATP) 2B1 was highest, followed by that of organic cation transporter 1, organic anion transporter 2, OATP1B1, OATP1B3, multidrug resistance-associated protein (MRP) 6, and MRP3. Stepwise multiple regression analysis demonstrated MRP4 mRNA level to be predicted with the greatest accuracy among 17 drug transporters. Of clinical variables entered into the prediction model for MRP4, hepatitis C virus (HCV) infection and liver cirrhosis were crucial factors affecting MRP4 mRNA and protein levels. Furthermore, HCV-related cirrhosis influenced the mRNA levels of 8 drug transporters besides MRP4. These findings indicate that HCV-related cirrhosis is a crucial factor affecting the expression of hepatic drug transporters, especially MRP4.

Organic anion transporter OAT1 is involved in renal handling of citrulline

Because citrulline plasma concentration is elevated in kidney failure, citrulline could be a biomarker of renal insufficiency, although the mechanism regulating its disposition in the kidney has not been clarified. In rat kidney slices, citrulline uptake was apparently Na+ dependent, saturable with Km 556 μM, and significantly inhibited by anionic (PAH) and cationic (TEA) compounds, but not by probenecid at 1 mM. Preincubation of kidney slices with glutarate increased citrulline uptake, while such an increase was not observed after preincubation of the slices in Na+-free buffer. This result suggested that a sodium-dependent dicarboxylate cotransporter is involved in citrulline uptake by rat kidney slices. In studies using transporter-overexpressing cells, human organic anion transporter 1 (OAT1) and rat Oat1 exhibited citrulline transport activity with Km values of 238 and 373 μM, respectively, while other OATs and organic cation transporters (OCTs) did not transport citrulline. Based on the relative activity factor method, the contribution of rat Oat1 to the overall uptake of citrulline in rat kidney slices was ∼70%. Moreover, the interaction among citrulline, PAH, and probenecid uptakes via rat Oat1 suggested that there are multiple functional sites on Oat1 and that the citrulline site may be distinct from the PAH and probenecid site. Thus OAT1/Oat1 appears to be one of the major contributors to renal basolateral uptake of citrulline, and impaired activities of these transporters may contribute substantially to the increase in plasma citrulline in renal failure. Accordingly, citrulline may be useful for diagnosis of kidney function as is creatinine.

Effects of drug transporters on volume of distribution

Recently, drug transporters have emerged as significant modifiers of a patient's pharmacokinetics. In cases where the functioning of drug transporters is altered, such as by drug-drug interactions, by genetic polymorphisms, or as evidenced in knockout animals, the resulting change in volume of distribution can lead to a significant change in drug effect or likelihood of toxicity, as well as a change in half life independent of a change in clearance. Here, we review pharmacokinetic interactions at the transporter level that have been investigated in animals and humans and reported in literature, with a focus on the changes in distribution volume. We pay particular attention to the differing effects of changes in transporter function on the three measures of volume. Further, trends are discussed as they may be used to predict volume changes given the function of a transporter and the primary location of the interaction. Because the liver and kidneys express the greatest level and variety of transporters, we denote these organs as the primary location of transporter-based interactions. We conclude that the liver is a larger contributor to distribution volume than the kidneys, in consideration of both uptake and efflux transporters. Further, while altered distribution due to secondary interactions at tissues other than the liver and kidneys may have a pharmacodynamic effect, these interactions, at least at the blood-brain barrier, do not appear to significantly influence overall distribution volume. The analysis provides a framework for understanding potential pharmacokinetic interactions rooted in drug transporters as they modify drug distribution.

Adaptive responses of renal organic anion transporter 3 (OAT3) during cholestasis

During cholestasis, bile acids are mainly excreted into the urine, but adaptive renal responses to cholestasis, especially molecular mechanisms for renal secretion of bile acids, have not been well understood. Organic anion transporters (OAT1 and OAT3) are responsible for membrane transport of anionic compounds at the renal basolateral membranes. In the present study, we investigated the pathophysiological roles of OAT1 and OAT3 in terms of renal handling of bile acids. The Eisai hyperbilirubinemic rats (EHBR), mutant rats without multidrug resistance-associated protein 2, showed higher serum and urinary concentrations of bile acids, compared with Sprague-Dawley (SD) rats (wild type). The protein expression level of rat OAT3 was significantly increased in EHBR compared with SD rats, whereas the expression of rat OAT1 was unchanged. The transport activities of rat and human OAT3, but not OAT1, were markedly inhibited by various bile acids such as chenodeoxycholic acid and cholic acid. Cholic acid, glycocholic acid, and taurocholic acid, which mainly increased during cholestasis, are transported by OAT3. The plasma concentration of beta-lactam antibiotic cefotiam, a specific substrate for OAT3, was more increased in EHBR than in SD rats despite upregulation of OAT3 protein. This may be due to the competitive inhibition of cefotiam transport by bile acids via OAT3. In conclusion, the present study clearly demonstrated that OAT3 is responsible for renal secretion of bile acids during cholestasis and that the pharmacokinetic profile of OAT3 substrates may be affected by cholestasis.

Analysis of regulatory polymorphisms in organic ion transporter genes (SLC22A) in the kidney

Organic cation transporters (OCTs) and organic anion transporters (OATs) (SLC22A family) play crucial roles in the renal secretion of various drugs. Messengar ribonucleic acid (mRNA) expression of transporters can be a key factor regulating interindividual differences in drug pharmacokinetics. However, the source of variations in mRNA levels of transporters is unclear. In this study, we focused on single nucleotide polymorphisms (SNP) in the promoter region [regulatory SNPs (rSNPs)] as candidates for the factor regulating mRNA levels of SLC22A. We sequenced the promoter regions of OCT2 and OAT1-4 in 63 patients and investigated the effects of the identified rSNPs on transcriptional activities and mRNA expression. In the OCT2 promoter region, one deletion polymorphism (-578_-576delAAG) was identified; -578_-576delAAG significantly reduced OCT2 promoter activity (p < 0.05), and carriers of -578_-576delAAG tend to have lower OCT2 mRNA levels, but the difference is not significant. There was no rSNP in the OAT1 and OAT2 genes. The five rSNPs of OAT3 and one rSNP of OAT4 were unlikely to influence mRNA expression and promoter activity. This is the first study to investigate the influences of rSNPs on mRNA expression of SLC22A in the kidney and to identify a regulatory polymorphism affecting OCT2 promoter activity.

Upregulation of rat renal cortical organic anion transporter (OAT1 and OAT3) expression in response to ischemia/reperfusion injury

BACKGROUND/AIMS

Renal organic anion transporters (OAT1 and OAT3) localized in the basolateral membrane mediate the uptake of organic anions from the blood into proximal tubules. This study aimed to examine the effects of renal ischemia/reperfusion injury (IRI) on the expression of cortical renal OAT1 and OAT3 and the functional impact.

METHODS

Male rats underwent a right nephrectomy and clamping of the left renal pedicle for 50 min or sham operation, followed by reperfusion for 1, 2, 4 and 6 days. The expression of OAT1 and OAT3 was detected by RT-PCR, immunohistochemistry and Western blot analysis. Na(+)-K(+)-ATPase activity was also estimated.

RESULTS

The renal clearance of para-aminohippurate was significantly decreased on day 1 in IRI rats compared with sham-operated rats and returned to normal when the tubular injury recovered. There were significant increases in the mRNA and protein levels of OAT1 and OAT3 in renal cortex homogenates and basolateral membranes on day 1 after IRI, while on days 2 and 4 after IRI, the renal expression of OAT1 and OAT3 decreased gradually but was still significantly higher than that of the sham-operated rats. The Na(+)-K(+)-ATPase activity in renal cortex homogenates decreased significantly on day 1 after IRI but gradually increased on days 2, 4 and 6.

CONCLUSIONS

Renal para-aminohippurate clearance was depressed in response to IRI; however, the expressions of renal cortex OAT1 and OAT3 were significantly elevated in the early stage of IRI which may have substantial impact on renal excretion of some drugs and toxic compounds.

Interaction and transport characteristics of mycophenolic acid and its glucuronide via human organic anion transporters hOAT1 and hOAT3

The immunosuppressant mycophenolate mofetil (MMF) is frequently administered with calcineurin inhibitors and corticosteroids to recipients of organ transplantations. However, the renal handling of the active metabolite mycophenolic acid (MPA) and 7-O-MPA-glucuronide (MPAG) has been unclear. The purpose of the present study was to assess the interaction of MPA and MPAG with the human renal organic anion transporters hOAT1 (SLC22A6) and hOAT3 (SLC22A8), by conducting uptake experiments using HEK293 cells stably expressing these transporters. MPA and MPAG inhibited the time-dependent uptake of p-[(14)C]aminohippurate by hOAT1 and that of [(3)H]estrone sulfate by hOAT3. The apparent 50% inhibitory concentration (IC(50)) of MPA for hOAT1 and hOAT3 was estimated at 10.7 and 1.5 microM, respectively. In the case of MPAG, the IC(50) values were calculated at 512.3 microM for hOAT1 and 69.1 microM for hOAT3. Eadie-Hofstee plot analyses showed that they inhibited hOAT1 noncompetitively and hOAT3 competitively. No inhibitory effects of tacrolimus, cyclosporin A and azathioprine on transport of p-[(14)C]aminohippurate by hOAT1 and of [(3)H]estrone sulfate by hOAT3 were observed. No transport of MPA by these transporters was observed. On the other hand, the uptake of MPAG into cells was stimulated by the expression of hOAT3, but not hOAT1. These findings propose the possibility that the administration of MMF decreases the renal clearance of drugs which are substrates of hOAT1 and hOAT3. Present data suggest that hOAT3 contributes to the renal tubular secretion of MPAG.

Hepatocyte nuclear factor-4{alpha} regulates the human organic anion transporter 1 gene in the kidney

Human organic anion transporter 1 (OAT1, SLC22A6), which is localized to the basolateral membranes of renal tubular epithelial cells, plays a critical role in the excretion of anionic compounds. OAT1 is regulated by various pathophysiological conditions, but little is known about the molecular mechanisms regulating the expression of OAT1. In the present study, we investigated the transcriptional regulation of OAT1 and found that hepatocyte nuclear factor (HNF)-4alpha markedly transactivated the OAT1 promoter. A deletion analysis of the OAT1 promoter suggested that the regions spanning -1191 to -700 base pairs (bp) and -140 to -79 bp were essential for the transactivation by HNF-4alpha. These regions contained a direct repeat separated by two nucleotides (DR-2), which is one of the consensus sequences binding to HNF-4alpha, and an inverted repeat separated by eight nucleotides (IR-8), which was recently identified as a novel element for HNF-4alpha, respectively. An electrophoretic mobility shift assay showed that HNF-4alpha bound to DR-2 and IR-8 under the conditions of HNF-4alpha overexpression. Furthermore, under normal conditions, HNF-4alpha bound to IR-8, and a mutation in IR-8 markedly reduced the OAT1 promoter activity, indicating that HNF-4alpha regulates the basal transcription of OAT1 via IR-8. This paper reports the first characterization of the human OAT1 promoter and the first gene in the kidney whose promoter activity is regulated by HNF-4alpha.

Renal Transport of Adefovir, Cidofovir, and Tenofovir by SLC22A Family Members (hOAT1, hOAT3, and hOCT2)

PURPOSE

The nephrotoxicity of the nucleotide antivirals adefovir, cidofovir and tenofovir is considered to depend on the renal tubular transport of them. Although it is known that the antivirals are substrates of the human renal organic anion transporter hOAT1 (SLC22A6), there is no information available on other organic ion transporters. The aim of the present study was to investigate whether the other renal organic anion transporter hOAT3 (SLC22A8) and organic cation transporter hOCT2 (SLC22A2) transport the antivirals.

MATERIALS AND METHODS

Uptake experiments were performed using HEK293 cells transfected with cDNA of the organic ion transporters.

RESULTS

The uptake of adefovir, cidofovir and tenofovir in monolayers stably expressing hOAT3 increased time-dependently, compared with control. Probenecid, a typical inhibitor of organic anion transporters, completely inhibited their transport. The amounts of the antivirals taken up by hOAT3 were much lower than those by hOAT1. The transient expression of hOCT2 did not increase uptake of the antivirals.

CONCLUSION

These results indicate that adefovir, cidofovir and tenofovir are substrates of hOAT3 as well as hOAT1, but that quantitatively hOAT1 is the major renal transporter for these drugs.

Gene expression and regulation of drug transporters in the intestine and kidney

Intestinal absorption and renal secretion of ionic drugs are controlled by a number of drug transporters expressed at the brush-border and basolateral membranes of epithelial cells. Over the last several years, considerable progress has been made regarding the molecular identification and functional characterization of drug transporters. Under some physiological and pathophysiological conditions, the expression and transport activity of drug transporters are changed, affecting the pharmacokinetics of substrate drugs. The regulation of transport activity in response to endogenous and exogenous signals can occur at various levels such as transcription, mRNA stability, translation, and posttranslational modification. Transcriptional regulation is of particular interest, because changes in transport activity are dynamically regulated by increases or decreases in levels of mRNA expression. The tissue-specific expression of drug transporters is also under transcriptional control, and recent studies using clinical samples from human tissues have revealed the expression profiles of drug transporters in the human body. The purpose of this research updates is to review the recent progress in the study of the gene expression and regulation of intestinal and renal drug transporters.

Impact of transporter-mediated drug absorption, distribution, elimination and drug interactions in antimicrobial chemotherapy

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis. Membrane transporters play important roles in determining the pharmacokinetic aspects of intestinal absorption, tissue distribution, and the urinary and biliary excretions of a wide variety of therapeutic drugs. The identification and characterization of transporters responsible for the transfer of nutrients and xenobiotics, including drugs, is expected to provide a scientific basis for understanding drug disposition, as well as the molecular mechanisms of drug-drug/drug-food/drug-protein interactions and inter-individual/inter-species differences. This review focuses on the influence of transporters on the pharmacokinetics of beta-lactam antibiotics, new quinolones, and other antimicrobial agents, as well as focusing on the drug-drug interactions associated with transporter-mediated uptake from the small intestine and transporter-mediated elimination from the kidney and liver.

Organic anion transporter family: current knowledge

Organic anion transporters (OATs) play an essential role in the elimination of numerous endogenous and exogenous organic anions from the body. The renal OATs contribute to the excretion of many drugs and their metabolites that are important in clinical medicine. Several families of multispecific organic anion and cation transporters, including OAT family transporters, have recently been identified by molecular cloning. The OAT family consists of six isoforms (OAT1 - 4, URAT1, and rodent Oat5) and they are all expressed in the kidney, while some are also expressed in the liver, brain, and placenta. The OAT family represents mainly the renal secretory and reabsorptive pathway for organic anions and is also involved in the distribution of organic anions in the body, drug-drug interactions, and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. In this review, current knowledge of and recent progress in the understanding of several aspects of OAT family members are discussed.

Human organic anion transporter 3 gene is regulated constitutively and inducibly via a cAMP-response element

Human organic anion transporter (OAT) 3 (SLC22A8) is localized to the basolateral membranes of renal tubular epithelial cells and plays a critical role in the excretion of anionic compounds. We previously reported that interindividual variation in the OAT3 mRNA level corresponded to interindividual differences in the rate of renal excretion of cefazolin. However, there is little information available on the molecular mechanisms regulating the gene expression of OAT3. Therefore, in the present study, we examined the transcriptional regulation of human OAT3. A deletion analysis of the OAT3 promoter suggested that the region spanning -214 to -77 base pairs was essential for basal transcriptional activity. This region contained a perfectly conserved cAMP-response element (CRE), and a mutation here led to a reduction in promoter activity. Electrophoretic mobility shift assays showed that CRE-binding protein (CREB)-1 and activating transcription factor (ATF)-1 bound to CRE. The activity of the OAT3 promoter was increased through the phosphorylation of CREB-1 and ATF-1 by treatment with 8-bromo-cAMP. This paper reports the first characterization of the human OAT3 promoter and shows that CREB-1 and ATF-1 function as constitutive and inducible transcriptional regulators of the human OAT3 gene via CRE.

Role of SLC xenobiotic transporters and their regulatory mechanisms PDZ proteins in drug delivery and disposition

Various types of xenobiotic (or drug) transporters have been recently identified to play important roles as barriers against toxic compounds and influx pumps to take up nutrients into the body. Since those xenobiotic transporters generally have wide range of recognition specificity and accept various types of compounds as substrates, localization and functional expression of such transporters could be one of the critical factors that affect the disposition and subsequent biological activity of therapeutic agents. Identification and characterization of drug transporters have given us a scientific basis for understanding drug delivery and disposition, as well as the molecular mechanisms of drug interaction and inter-individual/inter-species differences. To precisely understand pharmacological roles of the transporters in the body, it is also important to clarify molecular mechanisms involved in regulation of the transporters. As a first step to clarify the regulatory mechanisms that govern cell-surface expression and/or function of these transporters, recent researches have focused on PDZ (PSD-95/Discs-large/ZO-1) binding motif localized on carboxylic terminus of several types of xenobiotic transporters. Most of the transporters showing direct interaction potential with the PDZ domain-containing proteins are expressed on apical membranes in epithelial cells of kidney and/or small intestine, implying that such protein-protein interaction may play a role in apical localization of the transporters. In this mini-review article, we summarize importance of transporters and their regulatory mechanisms in drug delivery and disposition, focusing on several aspects of transporter-mediated drug targeting.